1. Field of the Invention
The present invention relates to an inverter apparatus for outputting a three-phase AC voltage having an arbitrary frequency from a DC voltage to thereby vary the speed of an induction motor, and particularly to an inverter apparatus which needs the detection of an output AC current, and an AC current detecting method therefor.
2. Description of the Related Art
As disclosed in Japanese Patent Application Laid-Open No. Hei 4(1992)-178197, a current detecting circuit for detecting motor""s instant phase currents of respective U, V and W phases may be provided to obtain the values of three-phase AC currents. Incidentally, the present publication shows that the instant currents of the respective phases are determined using this current detecting circuit and a current detecting circuit for detecting a DC output current, and the determined currents are added together to thereby obtain power consumed by a load.
If output AC currents of at least two phases or more can be theoretically detected, then output AC currents of three phases are obtained as the three-phase AC currents. Thus, output AC current values and a power factor angle are obtained.
Therefore, in the conventional inverter apparatus, a current sensor has been provided for at least two phase or more of output AC currents where it is necessary to carry out current detection good in accuracy.
A reduction in the size of the inverter apparatus has been advanced in recent years. An occupied physical size of a current sensor for detecting phase AC currents in the inverter apparatus has become innegligible.
While there is also known means for detecting only a current of a DC section and estimating the state of an AC current, only the current of the DC section allows detection of only an effective current component of the AC current and disables detection of an ineffective current component. Therefore, a true AC current value and a power factor angle cannot be obtained.
While the provision of an AC current detecting sensor and a DC current detecting sensor has been described in Japanese Patent Application Laid-Open No. 4(1992)-207992, these current sensors are provided as abnormal or improper signal detecting sensors. They are not provided for determining an output AC current value and a power factor.
An object of the present invention is to provide a technology for establishing a high-accuracy current detecting system using a less number of current sensors and implementing a reduction in the size of an inverter apparatus good in current detection accuracy.
If an AC current value |I1| and a power factor angle xcex8 can be obtained by performing detection of a current Idc of a DC section and detection of an AC current Iu, then the accuracy of current detection becomes satisfactory.
To this end, there is a need to provide either one of means for determining switching patterns of main elements and determining a current component of Idc and means for detecting a maximum value of Idc. Providing both the means for determining the switching patterns of the main elements and determining the current component of Idc and the means for detecting the maximum value of Idc allows more stable current detection.
Thus, it is feasible to reduce the number of current sensors, enhance the accuracy of detection of an AC current of an inverter apparatus, and bring the inverter apparatus into less size.